Dairy cows are significant investments for dairy farmers, and enormous efforts, such as animal breeding and artificial insemination, have been and continue to be invested in ensuring that the animals have high and sustained productivity, and that the milk produced are of high quality. Perhaps partially due to these breeding efforts for high productivity, fertility of the modern high-producing dairy cow has been decreasing for the last 30-50 years and has become a major concern of farmers and the dairy industry worldwide (Royal et al., 2000; Dobson et al., 2007; Dobson et al., 2008).
Previous studies have shown that a major subcomponent of infertility and higher incidence of late embryo mortality in dairy cattle is abnormal hormone activity causing different effects ranging from prolonged luteal activity to delayed ovulation (Lamming and Darwash 1998). However, recent studies have shown that low fertilization rates and embryonic loss—referring to death of embryos from the fertilization to completion of differentiation—seem to be the main factors contributing to infertility in dairy cattle (Santos et al., 2004; Morris and Diskin, 2008).
Although genetic factors are known to be involved in this decline of fertility (Royal et al., 2002), the discovery of specific genes has been challenging (Veerkamp and Beerda, 2007). The identification of genes with major effects on fertility would allow the implementation of gene-assisted selection to improve reproductive performance in dairy cows.
In addition, traditional breeding techniques involve the studying of sire progenies, and evaluating their traits including fertility or milk production ratings (transmitting abilities) to guide further breeding. This standard technique is time consuming and costly, requiring years to evaluate the true genetic value by progeny testing each bull. Many cows must be bred and give birth to offspring, and their fertility be measured, which may be inherently difficult. In case of mild production traits, the females must be raised, bred, allowed to give birth and finally milked for a length of time to measure their phenotypic traits.
Furthermore, selection based purely on phenotypic characteristics does not efficiently take into account genetic variability caused by complex gene action and interactions, and the effect of the environmental and developmental variants. There is thus a need for a method of genetically evaluating cattle to enable breeders to more accurately select animals at both the phenotypic and the genetic level.
Marker-assisted selection can lower the high cost of progeny testing currently used to improve sires, since young bull progeny could be evaluated immediately after birth, and young bulls that are determined by genetic testing to have undesirable markers would never be progeny tested or even prior to birth, for the presence/absence of the marker.
Thus, there is a need for the identification of genes with effects on fertility which would serve as genetic markers and allow the implementation of gene-assisted selection to improve reproductive performance in dairy cows.